Why not make the iron sights button show a "weapon aiming" animation for melee weapons? For example;
While holding 'iron sights' button with knife, the knife is moved so that the tip is center of screen.
With katana, the weapon is held in that 'badass stab stance' with tip in center (the one where the hilt is held at the side of the head with sword tip forward, like mitsurugi from soul calibur is commonly seen using).
With machete, weapon is held off camera with off hand stretched forward in a pose that allows for easy identification of center of screen (maybe at tip of thumb or something).
With claymore, sword is held in center of screen along the Z axis(?) (vertically) once again with tip at dead center.
And something else for axe and chainsaw- ran outta steam for ideas.
As long as iron sights is engaged the weapon is held in this pose- I figure most firing animations could play from these poses without too much visual discomfort. It would help to ascertain where to aim for headshots with melee weapons, at least for those of us who aren't already intimately familiar with this sort of thing